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Posted 17 Jun 2009

Writing Custom LINQ Extension Methods

, 17 Jun 2009
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Microsoft have done a great job with LINQ IMHO. However there are times when it might be handy to create your own LINQ extension methods.

Microsoft have done a great job with LINQ IMHO. However there are times when it might be handy to create your own LINQ extension methods. LINQ extension methods can be applied to any type provided that the source is of type IEnumerable<T>, so thats really the only requirement.

So how do we write an extension method. Well its quite simple. Here are 2 that I’ve written for a  IEnumerable<string> the first example takes a Predicate and the second one doesnt. This extension method will check whether a file name string is a valid image file, but you can do it for whatever your pupose is.


The only other thing to note is the cryptic “this” keyword used in the method signature. This means its being applied to the current IEnumerable<T> collection. Just always put it in your extension methods and you’ll be fine.

So how to use these extension methods, well providing your dealing with a  Enumerable<string> collection youll get the option coming up as an extension method.


And to actually use it you could use a standard LINQ query something like



This article, along with any associated source code and files, is licensed under The Code Project Open License (CPOL)


About the Author

Sacha Barber
Software Developer (Senior)
United Kingdom United Kingdom
I currently hold the following qualifications (amongst others, I also studied Music Technology and Electronics, for my sins)

- MSc (Passed with distinctions), in Information Technology for E-Commerce
- BSc Hons (1st class) in Computer Science & Artificial Intelligence

Both of these at Sussex University UK.


I am lucky enough to have won a few awards for Zany Crazy code articles over the years

  • Microsoft C# MVP 2016
  • Codeproject MVP 2016
  • Microsoft C# MVP 2015
  • Codeproject MVP 2015
  • Microsoft C# MVP 2014
  • Codeproject MVP 2014
  • Microsoft C# MVP 2013
  • Codeproject MVP 2013
  • Microsoft C# MVP 2012
  • Codeproject MVP 2012
  • Microsoft C# MVP 2011
  • Codeproject MVP 2011
  • Microsoft C# MVP 2010
  • Codeproject MVP 2010
  • Microsoft C# MVP 2009
  • Codeproject MVP 2009
  • Microsoft C# MVP 2008
  • Codeproject MVP 2008
  • And numerous codeproject awards which you can see over at my blog

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Comments and Discussions

GeneralTnx Pin
Bostjan Gajsler29-Apr-11 0:29
memberBostjan Gajsler29-Apr-11 0:29 
GeneralExtension method error Pin
wilfredogr16-Mar-11 4:00
memberwilfredogr16-Mar-11 4:00 
GeneralSome comments... Pin
Oleg Shilo17-Jun-09 14:41
memberOleg Shilo17-Jun-09 14:41 

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